1. Technical Field
The present invention generally relates to alkali metal thermal to electric conversion (AMTEC) cells and more particularly to a return channel for such a cell having a graded capillary structure for supporting the flow of condensed alkali metal in the cell which provides enhanced flow characteristics.
2. Discussion
An AMTEC cell is a thermally regenerative concentration cell typically utilizing sodium or potassium as a working fluid and a beta-alumina type solid electrolyte as an ion selective membrane. While throughout this description sodium is referred to as the working fluid, it is to be understood that other alkali metals are applicable to this invention. The electrolyte permits a nearly isothermal expansion of sodium to generate high-current/low voltage power at high efficiency. Most AMTEC cells employ at least one beta-alumina type solid electrolyte (BASE) element which is exposed to high-pressure sodium on one surface and low-pressure sodium on the opposite surface.
The BASE element's opposed surfaces are overlaid with permeable electrodes which are connected to each other through an external load circuit. Neutral sodium atoms incident on the BASE element's high pressure surface give up their electrons at one electrode (the anode). The resulting sodium ions pass through the element wall under the applied pressure differential, and the emerging sodium ions are neutralized at the other electrode (the cathode) by electrons returning from the external load. Thus, the pressure differential drives the sodium through the BASE element thereby creating an electrical current which passes through the external load resistance. One configuration for such an AMTEC cell utilizes BASE elements in the form of hollow cylindrical tubes in which the tube's inner surface supports the anode and the outer surface supports the cathode.
The neutral sodium atom vapor leaving the cathode flows through the space between the BASE elements and the cell wall until it condenses at the low-temperature condenser at one end of the cell. From there, the sodium condensate flows through an artery containing a fine pore wick commonly consisting of a packed metallic felt. The liquid sodium evaporates at the end of an evaporator wick which is coupled to the artery. The high-pressure sodium vapor is returned to the BASE elements through a common plenum at the opposite "hot" end of the cell.
Some cells employ multiple BASE tubes and are operated under conditions such that the sodium is in the vapor phase on both sides of the BASE elements to prevent shorting of the electrodes. In the cell configuration mentioned above, the inner surface of each BASE tube is exposed to high-pressure sodium vapor and the outer surface is exposed to low-pressure sodium vapor. The high-temperature evaporator near the hot end of the cell produces the high pressure and the low-temperature condenser at the cold end of the cell maintains the low-pressure.
In order to operate at high efficiency, the artery and evaporator, hereinafter referred to collectively as "the return channel", must support the recirculation of the alkali metal at a capillary pressure equal to or greater than the vapor pressure of the alkali metal at the hot end. As the alkali metal migrates along the length of the return channel, the vapor pressure changes in relation to the local cell temperature. That is, at lower temperature regions of the cell, the alkali metal vapor pressure is lower than it is at higher temperature regions of the cell.
To support the vapor pressure of the alkali metal, the capillary structure of the return channel creates a capillary pressure capable of sustaining the alkali metal flow. Conventional AMTEC cells employ a metallic felt or screen wick capillary structure with uniformly sized small pores or openings along the entire length of the return channel. This ensures that the capillary pressure at the hot end of the return channel is sufficient to support the flow of the alkali metal. However, the small pores at the cold end of the return channel typically result in a higher flow resistance which unduly restricts the flow of the alkali metal. This causes an undesirable pressure drop within the cell which adversely affects performance and leads to a corresponding low power output.
Accordingly, it is desirable to provide a return channel having a capillary structure for sustaining the flow of the alkali metal over a broad range of alkali metal vapor pressures. To accomplish this, a graded pore size capillary structure may be employed in the return channel having a small pore size, and corresponding high capillary pressure, at the hot end of the artery, and a larger pore size, and corresponding low capillary pressure and low flow resistance at the cold end of the artery. As such, the varying vapor pressure of the alkali metal is sustainable along the entire length of the return channel while minimizing the pressure drop of the working fluid traveling along the capillary structure. It is also desirable to provide a method of forming the graded capillary structure which is quick, reliable, and cost effective.